Papillary thyroid cancer (PTC), the most common endocrine malignancy, harbors several important oncogenic events, including BRAF mutation, Ras mutation, RET/PTC rearrangements, and methylation- induced silencing of the tumor suppressor gene RASSF1 A. Silencing of the thyroid-specific genes that are involved in iodide metabolism, such as the genes for thyroid-stimulating hormone receptor (TSHR) and sodium/iodide symporter (NIS), is responsible for the failure of some PTC patients to respond to radioiodine treatment. It is hypothesized that the oncogenic events, by activating the MAP kinase pathway, result in epigenetic alterations that are responsible for thyroid-specific gene silencing. To test this hypothesis, mutual exclusivity of the oncogenic events will first be tested in specific subtypes of PTC to support the concept that each oncogenic event is able to cause PTC through activating their shared MAP kinase pathway. Methylation status of thyroid-specific genes will be subsequently examined in the same tumors, with a focus on the TSHR and NIS genes, to define the relationship of oncogenic events with thyroid gene methylation. Human thyroid tumor cell lines with various oncogenic alterations, either naturally existing or experimentally created, will be used to study the functional relationship between the oncogenic events and thyroid gene methylation. Epigenetic histone modification and its relationship to DNA methylation in silencing thyroid genes and to oncogenic alterations will also be studied in cell lines. Specific kinase inhibitors and cell transfection with oncoproteins and siRNAs will be used to alter MAP kinase pathway activities in these studies. The role of BRAF mutation, the most common oncogenic event in PTC, and related MAP kinase pathway aberration in the epigenetic alteration and silencing of thyroid-specific genes will be the primary focus of these studies. We expect to discover important molecular information on the mechanisms of PTC pathogenesis and novel therapeutic targets for this most common endocrine cancer.